The rock could also be just to shore up the sheet piles to prevent them from collapsing over time. They're fine for a while as they were designed to be a cantileaved wall until the mat was constructed, but if left as is for a long period of time, (water) pressure would probably build up and this could cause Michigan Ave (or atleast the sidewalk) to become one with the excavation as I can't imagine it was designed to be a permanent wall.
Quote:
Originally Posted by SolarWind
March 10, 2020
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Also, FWIW in this picture from March (taken by Solar) I counted atleast 26 exposed drilled shafts.